This invention relates to the use of two or more polymeric materials combined together to form a composite, layered structure for use in microwave oven cookware or similar applications.
The suitability of plastics for use in microwave cookware depends on many factors. The characteristics which determine a polymer response to a microwave field are the dielectric properties, i.e. dielectric constant and dissipation factor. The dielectric constant is a measure of a material's ability to store an electric charge. The constant is the ratio of a condenser made from the material in question to the capacity of an identical condenser using air. The dielectric constant varies with frequency, temperature and humidity. The dissipation factor is the ratio of the inphase power to the 90.degree. out-of-phase power. The dissipation factor is the measure of the conversion of the reactive power, which is displayed in heat. The greater the dielectric constant and dissipation factor, the higher the dielectric loss, which results in greater and faster heatup in a microwave field.
Materials with softening temperatures of less than 100.degree. C. have not been able to withstand continued exposure to microwave energy in the so called "unloaded state" This state is when the cookware is placed in the oven without any food to be heated and the oven turned on. The minimum temperature reaction is the boiling point of water i.e. 212.degree. F. When in use, the oils, fats, grease and sugars can reach temperatures as high as 450.degree. F. These severe conditions preclude traditional thermoplastic materials such as polypropylene, polyethylene or polystyrene from having general purpose in microwave ovens.
The primary thermoplastic material used in microwave applications is polysulfone marketed by Union Carbide under the registered trademark UDEL. This resin is used because it has high continuous use temperature (300.degree. F.), steam resistance, long term resistance to creepe, good mechanical and electric properties, resistance to acids and alkalis in compliance with food contact regulations. Typical properties of polysulfone are as follows:
______________________________________ Density 1.24 gms/cc Tensile Strength 10,200 psi Tensile Elongation 50-100 % Modulus 390,000 psi Izod Impact 1.3 ft. lbs./in notch HDT 345 at 264 psi (.degree.F.) Dielectric Constant 3.07 72.degree. F. at 60-MH Dissipation Factor 0.0008-0.0034 72.degree. F. ______________________________________
The major disadvantage of polysulfone is its relatively high cost.
U.S. Pat. No. 3,821,015 issued June 28, 1974 to R. S. Feinberg discloses a process for improving the vapor barrier characteristics of a food handling utensil. The utensil is formed of paper or a thermoset crosslinked resin having a coating of a non-aqueous solution comprising a solvent and a linear polyamide-imide homopolymer or a polysulfone resin. After application of the coating, it is then heated to a temperature sufficient to dry the coating. The coating prevents the release of gases and vapors from the inner layer. These coating materials are semicrystalline and have distinct disadvantages, in that they are prone to induce crystallinity in repeated heat cycling in use and therefore have a short service life. U.S. Pat. No. 4,183,435, issued Jan. 15, 1980 to Thompson and Ihde discloses a polymeric multi-layered sheet material for making containers for food products. This structure has a thick cover of foamed high density polyethylene terephthalate. These core materials are semicrystalline and have limited dimensional stability.